1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device produced by depositing non-monocrystal silicon (called Si hereafter) film on insulating film formed on an insulating substrate made of glass or the like or a variety of substrates, for example a thin film transistor (TFT), a thin film diode (TFD) and a thin film integrated circuit containing transistors and diodes, especially a thin film integrated circuit for an active liquid crystal display (LCD).
2. Description of the Related Art
Recently, a semiconductor device in which TFTs are mounted on an insulating substrate made of glass or the like, for example an active liquid crystal display and an image sensor in which TFTs are used for activating pixels, has been developed.
Generally, thin film Si semiconductors are used for the TFTs used in the above-mentioned devices. The above-mentioned thin film Si semiconductor comprises two types of semiconductors, these being an amorphous Si semiconductor (a-Si) and a crystalline Si semiconductor. The amorphous Si semiconductor is used most generally because it can be readily produced at low temperatures by a vapor phase process, and is suitable for mass production; however, it is less conductive than a crystalline Si semiconductor.
Therefore, it is strongly desired that a fabricating method of TFTs made of a crystalline Si semiconductor should be established which can hereinafter obtain a higher speed characterstics. As a crystalline Si semiconductor, polycrystalline silicon, microcrystalline silicon, amorphous silicon containing a crystalline component and a semi-amorphous silicon which is in an intermediate condition of a crystal and an amorphous are well known.
As the production method of a thin film crystalline Si semiconductor, the following methods are known:
(1) Crystalline film is deposited directly.
(2) First, amorphous semiconductor film is deposited and next, it is crystallized by the energy of a laser beam.
(3) First, amorphous semiconductor film is deposited and next, it is crystallized by applying thermal energy for a long time (annealing).
However, even formation of a film which as a good semiconductive characteristic on the overall surface of a substrate by the method described in (1) is technically difficult. This method also has a cost problem in that a low-priced glass substrate cannot be used because the film is formed at a temperature of 600.degree. C. or more. Deposition of film which has a good characteristic at low temperatures by this method is difficult. As crystals grow perpendicularly to the substrate, film formed by this method is not suitable for TFTs which have flat conductivity.
For example, if an excimer laser used most generally at present is used in the method described in (2), this method has the problem that throughput is low because the area on which the laser beam is radiated is small. This method also has another problem in that the stability of the laser is not sufficient to form film evenly on the overall surface of a substrate having a large area. Further, this method requires a substrate to be heated and irradiated by a laser in a vacuum in order to crystallize well. Therefore, this method has the problem that throughput is limited.
The method described in (3) has the advantage that a substrate having a large area can be processed by this method compared with the methods described in (1) and (2). However, this method also requires high temperatures of 600.degree. C. or more to heat a substrate on which amorphous film is formed. If a low-priced glass substrate is used, the heating temperature must be lowered. Especially at present, LCD screens are becoming larger and larger and therefore, a large-sized glass substrate is required to be used for such large screens. If a large-sized glass substrate is used, a significant problem of shrinkage or distortion caused in the heating process essential for producing semiconductor devices and which deteriorates the precision of mask alignment occurs. Especially if a substrate made of N. 7059 glass manufactured by Corning Inc. used most generally at present is used, distortion occurs at a temperature of 593.degree. C. and significant deformation occurs in the prior crystallizing process due to heating. The heating time required for crystallization in the conventional process exceeds 20 to 30 hours and therefore, reduction of the time is also required together with reduction of the heating temperature.